SUMMARY Enterobacteriaceae are a family of Gram-negative bacteria that contains both commensals and pathogens relevant to human health. Among the most prominent pathogens of this family is non-typhoidal Salmonella enterica, a leading cause of infectious diarrhea worldwide. Key to this pathogen's success is its ability to elicit intestinal inflammation, a host response that creates a hostile gut environment where many commensals are depleted, but where Salmonella thrives, significantly increasing in abundance. During infection, pathogens must acquire essential metal nutrients such as iron, an element that is highly limited by the host. In preliminary studies, we found that the probiotic bacterium Escherichia coli Nissle 1917 can compete with Salmonella and other enteric pathogens in the inflamed gut by producing antibacterial peptides termed microcins. In particular, we found that Salmonella is susceptible to microcin M in iron-limited conditions and in the inflamed gut. The primary objective of this application is to elucidate the antimicrobial activity of E. coli Nissle's microcin M (MccM) in vitro and in vivo. Our central hypothesis is that conjugation to catecholate siderophores enables MccM to more selectively target bacteria that express specific siderophore receptors in the Fe-limited host environment, without affecting the gut microbiota at large. We plan to test our hypothesis and fulfill the objectives of this application by pursuing the following two Specific Aims. In Aim 1, we will determine whether siderophore conjugation and siderophore uptake machinery influences the antimicrobial activity of MccM, and we will identify putative cellular targets of MccM activity by screening mutant libraries. Investigate the mechanisms underlying the selectivity of MccM antimicrobial activity. In Aim 2, we will ascertain the antimicrobial activity of MccM against non-typhoidal Salmonella in vivo, and we will determine whether administration of purified MccM perturbs the gut microbiome.This work may lead to future development of microcins as therapeutics to limit colonization and transmission of non-typhoidal Salmonella, and possibly other enteric pathogens and pathobionts, in an environment that is otherwise favorable to their growth.